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Taking a break: paused accretion in the symbiotic binary RT Cru

A. Pujol, G. J. M. Luna, K. Mukai, J. L. Sokoloski, N. P. M. Kuin, F. M. Walter, R. Angeloni, Y. Nikolov, R. Lopes de Oliveira, N. E. Nuñez, M. Jaque Arancibia, T. Palma, L. Gramajo

Submitted on 23 November 2022

Figures are fetched from the INSPIRE database at: https://inspirehep.net/literature/2514250

Schematic view of the datasets used in this study, where filler black squares highlight the years of the observation obtained from a given instrument, database or literature. Blue squares mark the year of the optical spectroscopic data.
Figure 1: Schematic view of the datasets used in this study, where filler black squares highlight the years of the observation obtained from a given instrument, database or literature. Blue squares mark the year of the optical spectroscopic data.
Long-term optical to hard-X-ray variability of RT~Cru. {\it Panel a:} RT~Cru optical {\it B}- and {\it V}-band light curves with measurements from \cite[][black plus signs]{1994A&AS..106..243C}, ASAS (gray dots), AAVSO (black crosses for V band, red circles for B band), and ASAS-SN (open circles). {\it Panel b:} B-V curve from AAVSO data. {\it Panel c:} \swift/UVOT UVM2 (open circles) and UVW2 blue dots light curves. {\it Panel d:} Light blue dots are \swift/BAT 14--50 keV measurements with 100-day bins (with 1$\sigma$ error bars). The maximum in the BAT light curve around 2011-2013 is due to an increase in the accretion rate which did not increase enough to reach the level where the boundary layer becomes optically thick and a super-soft X-ray component arise \citep[see][]{2018A&A...619A..61L}. Between late 2019 and late 2020, RT~Cru was not detected by \swift/BAT in 100-days bins.{\it Panel e:} \swift/XRT 0.3-10 keV count rate in log scale to highlight the fade in flux during 2019. {\it Panel f:} \swift/XRT softness ratio. {\it Panel g:} H$\alpha$ (blue dots) and adjacent continuum (yellow dots) flux evolution. {\it Panel h:} He I 6678 \AA~ (red dots) and adjacent continuum (black dots) flux evolution. Starting around mid-2017, the B-V color became more red for a few years (panel {\it b}) while the hard X-ray flux dipped (panel {\it d}). The UV and soft X-ray emission (panels {\it c} and {\it e}) also dipped at around that same time.
Figure 2: Long-term optical to hard-X-ray variability of RT~Cru. {\it Panel a:} RT~Cru optical {\it B}- and {\it V}-band light curves with measurements from \cite[][black plus signs]{1994A&AS..106..243C}, ASAS (gray dots), AAVSO (black crosses for V band, red circles for B band), and ASAS-SN (open circles). {\it Panel b:} B-V curve from AAVSO data. {\it Panel c:} \swift/UVOT UVM2 (open circles) and UVW2 blue dots light curves. {\it Panel d:} Light blue dots are \swift/BAT 14--50 keV measurements with 100-day bins (with 1σ error bars). The maximum in the BAT light curve around 2011-2013 is due to an increase in the accretion rate which did not increase enough to reach the level where the boundary layer becomes optically thick and a super-soft X-ray component arise \citep[see][]{2018A&A...619A..61L}. Between late 2019 and late 2020, RT~Cru was not detected by \swift/BAT in 100-days bins.{\it Panel e:} \swift/XRT 0.3-10 keV count rate in log scale to highlight the fade in flux during 2019. {\it Panel f:} \swift/XRT softness ratio. {\it Panel g:} Hα (blue dots) and adjacent continuum (yellow dots) flux evolution. {\it Panel h:} He I 6678 \AA~ (red dots) and adjacent continuum (black dots) flux evolution. Starting around mid-2017, the B-V color became more red for a few years (panel {\it b}) while the hard X-ray flux dipped (panel {\it d}). The UV and soft X-ray emission (panels {\it c} and {\it e}) also dipped at around that same time.
\swift/XRT count rate light curve in the 0.3--10 keV energy range. The colored boxes show the observations that were combined to obtain the spectra analyzed in Section \ref{sec:swfitXRT}.
Figure 3: \swift/XRT count rate light curve in the 0.3--10 keV energy range. The colored boxes show the observations that were combined to obtain the spectra analyzed in Section \ref{sec:swfitXRT}.
{\it TESS} light curves of the observations obtained during Sector 11 in 2019 with a 30 m cadence (top row), and the light curves from observations obtained during Sectors 37 and 38 in 2021 with a 10 m cadence (bottom row). The vertical dashed line in the lower left-hand plot marks the separation between Sectors 37 and 38. Right panels: Each portion of the light curve of each sector has a Savitzky-Golay filter (red line in the left panels) subtracted to enable the study of short-term, flickering-type variability. Right-hand panels show the light curves after the SG filter was subtracted. Variability on time scales shorter than 10 min (in Sectors 37 and 38) or 30 min (in Sector 11) cannot be detected. In addition, the measurement errors (0.001 mag) are larger than the observed dispersion; no short-term variability can be detected.
Figure 4: {\it TESS} light curves of the observations obtained during Sector 11 in 2019 with a 30 m cadence (top row), and the light curves from observations obtained during Sectors 37 and 38 in 2021 with a 10 m cadence (bottom row). The vertical dashed line in the lower left-hand plot marks the separation between Sectors 37 and 38. Right panels: Each portion of the light curve of each sector has a Savitzky-Golay filter (red line in the left panels) subtracted to enable the study of short-term, flickering-type variability. Right-hand panels show the light curves after the SG filter was subtracted. Variability on time scales shorter than 10 min (in Sectors 37 and 38) or 30 min (in Sector 11) cannot be detected. In addition, the measurement errors (0.001 mag) are larger than the observed dispersion; no short-term variability can be detected.
The SMARTS optical spectrum of RT~Cru obtained on 2019-03-17 (black line) and the UVOT spectra obtained on 2019-01-15 (green line) and 2020-08-12 (gray line), corrected for the interstellar extinction using E(B-V)=0.53. The blue line is a template spectrum of a M6III giant from \citet{2015RAA....15.1154Z}.
Figure 5: The SMARTS optical spectrum of RT~Cru obtained on 2019-03-17 (black line) and the UVOT spectra obtained on 2019-01-15 (green line) and 2020-08-12 (gray line), corrected for the interstellar extinction using E(B-V)=0.53. The blue line is a template spectrum of a M6III giant from \citet{2015RAA....15.1154Z}.
Selected optical spectra taken with the SMARTS/Chiron telescope from March 2019 through March 2022 (see the complete list of spectra in Table \ref{tab:chironlog}). Since March 2019, H$\alpha$ is detected in emission, while its flux increased monotonically (see Panel {\em g} in Fig. \ref{fig1}). Other lines such as N[II] 6584 \AA~ were also missing in 2019 and reappeared in emission afterwards.
Figure 6: Selected optical spectra taken with the SMARTS/Chiron telescope from March 2019 through March 2022 (see the complete list of spectra in Table ). Since March 2019, H is detected in emission, while its flux increased monotonically (see Panel {\em g} in Fig. ). Other lines such as N[II] 6584 \AA~ were also missing in 2019 and reappeared in emission afterwards.
{\em Top}: {\it TESS} light curve of the second half of Sector 38. {\em Bottom}: LS power spectrum (green line) with red noise model (black line) and detection levels at 3$\sigma$ (orange).
Figure 7: {\em Top}: {\it TESS} light curve of the second half of Sector 38. {\em Bottom}: LS power spectrum (green line) with red noise model (black line) and detection levels at 3 (orange).
UVW2 \swift/UVOT and \xmm/OM light curves with 120 s bins. The UVOT light curves from dates outside the low-flux optical state (years 2016 and 2021; top and bottom panels) show strong variability whereas the light curve from the 2019 \xmm/OM observation (middle panel) shows a much fainter source with no significant variability.
Figure 8: UVW2 \swift/UVOT and \xmm/OM light curves with 120 s bins. The UVOT light curves from dates outside the low-flux optical state (years 2016 and 2021; top and bottom panels) show strong variability whereas the light curve from the 2019 \xmm/OM observation (middle panel) shows a much fainter source with no significant variability.
X-ray spectral evolution between 2005 and 2022, as revealed by \xmm~ and \swift. The solid curves in the top portion of each panel are the best-fit models, while dotted lines shows the contribution of each spectral component in the cases where more than one spectral component was needed to model the spectrum (see Section \ref{sec:xfit}). Each panel lists the accretion rate through the optically thin boundary layer ($\dot{M}$, in units of 10$^{-9}$ \ms yr$^{-1}$). The resulting fit parameters are listed in Table \ref{tab:xray}. The spectra taken in 2019 with \swift\ and \xmm\ clearly show a decrease both in absorption and accretion rate.
Figure 9: X-ray spectral evolution between 2005 and 2022, as revealed by \xmm~ and \swift. The solid curves in the top portion of each panel are the best-fit models, while dotted lines shows the contribution of each spectral component in the cases where more than one spectral component was needed to model the spectrum (see Section ). Each panel lists the accretion rate through the optically thin boundary layer (, in units of 10 \ms yr). The resulting fit parameters are listed in Table . The spectra taken in 2019 with \swift\ and \xmm\ clearly show a decrease both in absorption and accretion rate.
Light curves of our short-term optical monitoring of RT~Cru, obtained with the $HSH$ telescope. R (red stars), V (light blue triangles) and B (blue circles) magnitudes are displayed, while error bars are smaller than the symbols. Cadence and observing length are listed in Table ~\ref{tab-flickering}
Figure 10: Light curves of our short-term optical monitoring of RT~Cru, obtained with the telescope. R (red stars), V (light blue triangles) and B (blue circles) magnitudes are displayed, while error bars are smaller than the symbols. Cadence and observing length are listed in Table ~
{\it Same as Fig. \ref{fig.rtcru.hsh}}.
Figure 11: {\it Same as Fig. \ref{fig.rtcru.hsh}}.
{\it Same as Fig. \ref{fig.rtcru.hsh}}..
Figure 12: {\it Same as Fig. \ref{fig.rtcru.hsh}}..
Light curves of our short-term optical monitoring of RT~Cru, obtained with the $Gemini$-South telescope. (see Table ~\ref{tab-flickering}).
Figure 13: Light curves of our short-term optical monitoring of RT~Cru, obtained with the -South telescope. (see Table ~).
Light curves of our short-term optical monitoring of RT~Cru, obtained with the $DuPont$ telescope. Blue circles show B-band differential magnitudes while black circles show U-band magnitudes (see Table ~\ref{tab-flickering}).
Figure 14: Light curves of our short-term optical monitoring of RT~Cru, obtained with the telescope. Blue circles show B-band differential magnitudes while black circles show U-band magnitudes (see Table ~).
Light curves of our short-term optical monitoring of RT~Cru, obtained with the $Swope$ telescope. Blue circles show B-band differential magnitudes while black circles show U-band magnitudes (see Table ~\ref{tab-flickering}).
Figure 15: Light curves of our short-term optical monitoring of RT~Cru, obtained with the telescope. Blue circles show B-band differential magnitudes while black circles show U-band magnitudes (see Table ~).
$V$-band light curves of RT~Cru, obtained from the AAVSO database (see Table ~\ref{tab-flickering}).
Figure 16: -band light curves of RT~Cru, obtained from the AAVSO database (see Table ~).